THE MATRIX OF THE DIAMOND' 23
chrome-diopside, the composition and optical characters being identical.1
Chrome-diopside
gives birth to granular calcite, which always accompanies it when
decomposed. When there is any doubt in the section as to whether a
mineral is bronzite or chrome-diopside, the presence of calcite on its
edges and in its cracks is often a sufficient criterion for diopside.
The composition is probably that of the pure diopside molecule, CaO,
MgO, 2Si02.
Chrome-diopside
is well known to occur with enstatite in dunite, in lherzolite and in
other peridotites and serpentines, and in the 'olivine bombs' which
are enclosed in basalt. Descloiseaux has described a chrome-diopside in
the platiniferous (and diamantiferous ?) peridotite of Nischne-Tagilsk,
Urals.
Fluid
or glass inclusions, resembling those in the olivine, were noticed in
cleavage fragments of the chrome-diopside, which were otherwise pure.
The
fact has already been pointed out that bronzite and chrome-diopside
occur sometimes enclosed in olivine, and sometimes surrounded by what
seems to be a fusion zone. This fusion zone or growth zone has a
peculiar worm-like radiating structure, which may be compared with that
of the kelyphite rim2 around garnets in serpentine, or the radial zones around olivine in certain norites3 and gabbros, or the so-called granophyric4 or pegmatitics structure in some quartz-porphyries, or the structure in the chondri of meteorites.6
The principal mineral in these zones is a colourless substance, in
short worm-like forms, with a high index of refraction, high
